1 / 18

Why is the Night Sky Dark?

Why is the Night Sky Dark?. ‘Cosmology’ Studies of the universe as a whole… Today… Brief history of ideas (Early Greeks  Big Bang ) The e x pa n d in g universe (Hubble, Relativity, density & destiny) An alternative theory: The ‘Steady State ’ model.

latika
Download Presentation

Why is the Night Sky Dark?

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Why is the Night Sky Dark? ‘Cosmology’ • Studies of the universe as a whole… Today… • Brief history of ideas (Early Greeks  Big Bang) • The expanding universe (Hubble, Relativity, density & destiny) • An alternative theory: The ‘Steady State’ model Our Evolving Universe

  2. A brief history of Ideas…Are we in a special place? • Early Greeks (BC) • ‘Geocentric’ - Earth centred • Copernicus (1543 AD) • ‘Heliocentric’ - Sun centred • Newton (1600s AD) • Governed by law of gravity Geocentric Heliocentric • From Newton on (1600s  now) • We’re NOT in a special place. • The universe looks the same from everywhere and in all directions (‘Cosmological Principle’). The Cosmological Principle Our Evolving Universe

  3. A brief history of Ideas…Why is the night sky dark? Olbers’ Paradox • Pre-1930’s the universe was widely thought to be infinite, eternal and static… • But if so… ‘Why is the night sky dark?’ (Heinrich Olbers ~ 1800’s AD) • Expect: • Sky as bright as surface of average star! • Overall temperature of sky > 3000K! • Something is WRONG! Our Evolving Universe

  4. A brief history of Ideas…Einstein’s Universe • Special & General Relativity (Early 1900s) • Revolutionary new descriptions of space, time, gravity and the universe. • Einstein’s model of the Universe assumed: • The Cosmological Principle • The universe isSTATIC • He invented the cosmological constant (Λ) to allow static universe in General Relativity • Note that he could have predicted expansion! Our Evolving Universe

  5. H + K 300 Mpc 15,000 km/sec Virgo 430 Mpc 22,000 km/sec Ursa Major Hubble’s 1929 data Velocity (km/sec) Distance (Mpc) A brief history of Ideas…Hubble’s Expanding Universe! • 1929 -Edwin Hubble’s analysis of light spectra of distant galaxies revealed the universe is expanding… • Doppler effect shows: Redshift  Velocity • Hubble’s Law: Velocity  Distance • Einstein concedes: The universe IS expanding. • The introduction of was his “greatest blunder”! Our Evolving Universe

  6. T = 0 Now ‘Inflation’ ‘Nucleosynthesis’ Galaxy formation Particle/photon Soup ‘Recombination’ A brief history of Ideas…Current Thinking: The Big Bang The universe is expanding & cooling after an initial ‘explosion’ ~14 billion years ago. ~ 3K Very hot Next lecture - ‘The 1st 400,000 years’ Our Evolving Universe

  7. The Observable Universe The Observable Universe How does the Big Bang theory solve Olbers’ paradox? • Olbers’ paradox assumes the Universe is Infinite, Eternal and Static. 1. The Big Bang universe is NOT eternal. • So the observable universe is not infinite. 2. The Big Bang universe is NOT static. • Light from distant objects cools as the universe expands Our Evolving Universe

  8. Hubble’s findings revisited. • Modern measurements of distant galaxies confirm Hubble’s findings • Hubble’s Law: V = H0D • H0 = Hubble’s Constant • Recent estimates: H0 ~ 70 km.s-1.Mpc-1 1996 type 1 SNe The Expanding Universe • Does Hubble’s law go against the cosmological principle? • No - the appearance of everything receding from US would be the same anywhere.. Our Evolving Universe

  9. Expansion slowed by gravity age The expanding universe…The age of the universe • Hubble’s law can be used to calculate the age of the universe: • If expansion (V) is constant: Age = D0/V = 1/H0 • If H0 ~70 km.s-1.Mpc-1 Age ~ 14 billion years D0 V=H0D0 Separation of galaxies (D) • Age is less if expansion is slowed by gravity • With Λ=0,would expect age ~ 9 billion years • (With Λ>0, age goes back to~ 14 billion years – see later) now Time (t) age Our Evolving Universe

  10. Surface of balloon analogy The expanding universe…What does the expansion look like? • Not like a normal explosion • Not IN a particular place AT a particular time. • Big bang thought to be the beginning of time itself. • Expansion OF space not WITHIN space. Raisins in bread analogy • Hard (very hard) to visualise. Our Evolving Universe

  11. General Relativity (1915)… Concerns motion, space-time, gravity and mass. • Mass WARPS Space-time. • Warping perceived as gravity The expanding universe…Einstein’s Relativity Concerns motion, space, time & the speed of light. Special Relativity (1905)… SPACE-TIME • The speed of light (c) is always constant (3108 m s-1) • Nothing can go faster than c. • Space & time are not independent. • Mass & energy are related: E = mc2 Our Evolving Universe

  12. The expanding universe…Evidence for General Relativity • General Relativity predicts light is bent by massive objects ‘Gravitational Lensing’ Our Evolving Universe

  13.  = c  > c  < c ‘Critical Density’ c= ‘Flat’ ‘Closed’ ‘Open’ Size of the Universe Infinite Finite Infinite The expanding universe…Curvature of the Universe • General relativity says: • Space-time in the universe is CURVED! • Curvature depends on mass density (). If Our Evolving Universe

  14. In simple models (Λ=0): • If  < c: (Open Universe) Universe will expand forever  < c  = c • If  = c: (Flat universe) Expansion decelerates towards zero  > c • If > c: (Closed Universe) Universe will eventually collapse (the Big Crunch!) The expanding universe…What about the future? • Fate of the universe is dependent on density () & critical density (c) But... BEWARE OF THE COSMOLOGICAL CONSTANT! (See later!) Density is Destiny! =0 Separation of galaxies (D) Time (t) Which is it? 3rd lecture - ‘Vital Statistics of the Universe’ Our Evolving Universe

  15. Summary so far… • Brief history of ideas.. • The Cosmological Principle: The universe looks the same in all directions and in any location. • Olbers’ Paradox: If the universe is infinite, eternal and static - ‘Why is the night sky dark?’ • Now believe the universe is in a state of expansion and cooling after an initial ‘Big Bang’ ~14 billion years ago. Our Evolving Universe

  16. Summary so far… • The expanding universe: • Distant objects are receding according to Hubble’s law (V=H0D). • The Big Bang is not like a normal explosion. • Expansion OF (not WITHIN) space • Big bang = beginning of time • Einstein’s Relativitydescribes gravity, space, time and the structure of the universe. • The Age and Fate of the universe dependant on density () Our Evolving Universe

  17. Fred Hoyle (1915-2001) • The Steady State Model… • There was no big bang • The universe has always looked same as it does now. • It is infinite but expanding • Mass is created as space expands new mass An alternative model?The Steady State Model • So far we know… • Distant objects receding according to Hubble’s Law • Structure of universe described by General Relativity. • Is the Big Bang the only Model that fits? Our Evolving Universe

  18. Yes… Light is cooled by expansion An alternative model?The Steady State Model • Does the Steady State model also solve Olbers’ Paradox? • Big Bang or Steady State? • Any evidence for change over time? • Any evidence for hot beginnings? 3rd lecture - ‘Vital statistics of the Universe’ Our Evolving Universe

More Related